In general, H.264/MPEG-4 AVC (hereafter, referred to as H.264) is used as a method to encode video images to be stored in compressed format. According to H.264 (NPL 1:ITU-T H.264 (06/2011) Advanced video coding for generic audiovisual services), multiple profiles defining encoding processing restrictions are defined. Among these, the High 10 profile supports images with bit depths between 8 to 10 bits for both luminance and chrominance.
The High 10 profile in H.264 not only supports images with bit depths between 8 to 10 bits for both luminance and chrominance, but also supports images with difference between the bit depths for the luminance and chrominance.
According to H.264, the applicable range regarding quantization parameters corresponding to the image bit depths are decided. For example, when the image luminance signal has a bit depth of 8 bits, the values for the quantization parameter is between 0 to 51, and when the bit depth is 10 bits, the values for the quantization parameter are between −12 and 51. Thus, the configuration lowers the minimum value of the quantization parameter when the bit depth is increased. The difference between the quantization parameter corresponding to the luminance signal (hereafter, referred to as luminance quantization parameter) and the quantization parameter corresponding to the chrominance signal (hereafter, referred to as chrominance quantization parameter) is defined by chroma_qp_index_offset code. When the luminance quantization parameter is 20, and the chroma_qp_index_offset code is five, for example, the chrominance quantization parameter is 25. However, the range of values possible for the chroma_qp_index_offset code is between −12 to +12. For this reason, the difference between the luminance quantization parameter and the chrominance quantization parameter may not be particularly significant with this specification.
When the luminance signal is 10 bits and the chrominance signal is 8 bits, the minimum value of the luminance quantization parameter is −12 and the minimum value of the chrominance quantization parameter is zero. For this reason, when the luminance quantization parameter is −12, the value for the chrominance quantization parameter is restricted to zero as the maximum value of the chroma_qp_index_offset code is 12. That is to say, when the bit depths of the luminance signal and the chrominance signal are different, the range of the usable chrominance quantization parameter value becomes drastically reduced depending on the value of the luminance quantization parameter.
It has been found desirable to increase the usable value of the chrominance quantization parameter when the bit depths between the luminance signal and the chrominance signal are different, to enable image encoding and decoding with higher levels of image quality control of chrominance signals.